Photochromic, phototropic, or reversibly darkening glasses, as such have been variously termed, had their genesis in U.S. Pat. No. 3,208,860. That patent discloses glass articles containing silver halide crystals which impart the photochromic properties thereto. Whereas numerous base glass compositions to provide matrices for those crystals have been described and claimed in the patent literature, the alkali metal oxide (R.sub.2 O) aluminoborosilicate system has seen by far the greatest service in commercial applications.
The preferred base compositions of U.S. Pat. No. 3,208,860 consisted essentially, in weight percent on the oxide basis, of about 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3, 40-76% SiO.sub.2, and the remainder R.sub.2 O, wherein R.sub.2 O consisted of at least one member in the indicated proportion selected from the group of 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O, the sum of R.sub.2 O+Al.sub.2 O.sub.3 +B.sub.2 O.sub.3 +SiO.sub.2 constituting at least 85% of the total composition. Photochromic behavior was developed through the inclusion of at least one halide in a minimum effective amount of 0.2% Cl, 0.1% Br, and 0.08% I, and the inclusion of silver in at least the minimum effective amount of silver in at least the effective halide, 0.05% where bromide is the effective halide, and 0.03% where iodide is the effective halide.
Subsequent disclosures have varied the ranges of both the base glass composition and the silver halides in order to endow the products resulting therefrom with certain desired products. The following patents are representative of such disclosures, the ranges being defined in weight percent.
______________________________________ U.S. Pat. No. U.S. Pat. No. U.S. Pat. No. 4,018,965 4,130,437 4,190,451 ______________________________________ SiO.sub.2 54-66 57.1-65.3 20-65 Al.sub.2 O.sub.3 7-15 9.6-13.9 5-25 B.sub.2 O.sub.3 10-25 12-22 14-23 Li.sub.2 O 0.5-4 1-3.5 0-2.5 Na.sub.2 O 3.5-15 3.7-12 0-9 K.sub.2 O 0-10 0-5.8 0-17 Li.sub.2 O + Na.sub.2 O + 6-16 6-15 -- K.sub.2 O PbO 0-3 0-1.25 0-0.5 Ag 0.1-1 0.12-0.24 0.15-0.3 Cl 0.1-1 0.2-1 0.1-0.25 Br 0-3 0.06-0.25 0.1-0.2 CuO 0.008-0.16 0.002-0.02 0.004-0.02 F 0-2.5 0-2.5 0-2 Cs.sub.2 O -- -- 0-6 Li.sub.2 O + Na.sub.2 O + -- -- 8-20 K.sub.2 O + Cs.sub. 2 O ______________________________________ U.S. Pat. No. U.S. Pat. No. U.S. Pat. No. 4,168,339 4,358,342 4,407,966 ______________________________________ SiO.sub.2 54-66 55-60 56-60 Al.sub.2 O.sub.3 7-16 9-10 6-9 B.sub.2 O.sub.3 10-30 19-20.5 18-21 Li.sub.2 O 0-4 2-2.5 &gt;2.5-3.5 Na.sub.2 O 3-15 2-3 0.5-2.5 K.sub.2 O 0-10 6-7 5-7 PbO 0.4-1.5 0.1-0.25 0.1-0.15 ZrO.sub.2 -- -- 3.75-5 Ag &gt;0.03-1 0.1-0.15 &gt;0.15-0.25 Cl 0.5-1.2 0.3-0.5 0.2-0.35 Br 0.2-0.5 0.05-0.15 0.075-0.15 CuO 0.008-0.03 0.0065-0.01 0.004-0.02 CeO.sub.2 -- -- 0-0.5 F 0.2-0.5 -- -- ______________________________________ U.S. Pat. No. U.S. Pat. No. 4,549,894 4,608,349 ______________________________________ SiO.sub.2 5-61 54-58 Al.sub.2 O.sub.3 5-11 7-8 B.sub.2 O.sub.3 18-21 18-22 Li.sub.2 O 1.5-3 3.75-4.5 Na.sub.2 O 2-5 0-1 K.sub.2 O 4.5-8 5.5-7.5 ZrO.sub.2 -- 2-4.5 Ag &gt;0.1-&lt;0.2 0.20-0.33 Cl 0.2-0.6 0.30-0.50 Br 0.05-0.2 0.04-0.12 CuO 0.003-0.015 0.007-0.012 CeO.sub.2 0.1-1.0 -- As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3 0.1-1.5 0-0.20 ______________________________________
The generation of the silver halide crystals necessary to impart photochromic behavior can take place at a temperature within the transformation range of a glass. (The transformation range has been generally defined as that temperature at which a molten mass becomes an amorphous solid, that temperature commonly being deemed as residing in the vicinity of the annealing point of a glass.) Hence, the glass must exhibit sufficient viscosity to permit the development and growth of crystals. Nevertheless, it is well recognized that crystal growth occurs more rapidly as the temperature is raised, with the concomitant reduction in the viscosity of the glass, such that in commercial practice temperatures approaching and even somewhat exceeding the softening point of the glass are customarily employed in the heat treatment thereof to induce crystal growth. Accordingly, the heat treatment temperature utilized will be dependent upon the base composition of the particular glass. For example, in the alkali metal aluminoborosilicate-based photochromic glasses of the types disclosed in the patent literature, temperatures between about 600.degree.-800.degree. C., normally about 640.degree.-700.degree. C., are used.
It will be self-evident that temperatures in the vicinity of the softening point of the glass lead to thermal deformation of the glass articles. The most widespread commercial application of photochromic glass has been in the production of ophthalmic lenses. In pressing ophthalmic blanks, a curved former is employed and the glass sags to a desired curve. Thus, thermal deformation comprises part of the fabrication and crystallization process. In contrast, when heat treating sheet glass for such applications as sky lights and windows to develop silver halide crystals therein, thermal deformation has presented a very formidable problem.
Accordingly, the primary objective of the instant invention was to devise a means for heat treating potentially photochromic glass sheet to cause the in situ generation of silver halide crystals, wherein the sheet will be essentially free of thermal deformation and surface defects, and the photometric properties will be essentially uniform throughout the area of the sheet. As employed herein, "potentially photochromic glass" defines a glass containing silver and halide ions which, upon heat treatment, will crystallize in situ, thereby imparting photochromic behavior to the glass. In general, the thickness of the glass sheet will range about 0.5-3 mm, preferably about 1-2 mm.